The field of the present disclosure relates generally to gas flare operations and more specifically, to methods and systems for controlling the operating parameters of a gas flare used in industrial plants to burn flammable gasses.
At least some known flare control systems use a gas chromatograph (GC) to determine a flare gas composition and calculate a BTU value of the flare gas. This method does not directly measure a heating value of the flare gas, but rather speciates the sample and determines the heating value by summing the products of each components' heating value and the molecular fraction of each component. The GC, however, is not a continuous measuring instrument and has a slow response time that can take many minutes for a BTU determination for each gas sample. In addition, the data received from the GC is many minutes old, and, as is common in flare systems, the composition of a flare vent gas can be subject to fast fluctuations and variances.
At least some other known flare control systems use a calorimeter to calculate a BTU value of the flare gas. A calorimeter mixes and burns a sample of the flare gas with air or another fuel. At least some known calorimeters regulate the flow of air to maintain a constant exhaust temperature. The air flow variation provides an input that can be used to calculate a heating value of the flare gas. Some other calorimeters measure excess oxygen content after combustion of the flare gas sample. The residual oxygen content provides an input that can be used to calculate a heating value of the flare gas. Such calorimeters, however, can have a response time greater than one minute or more for a BTU determination for each gas sample. In addition, a calorimeter cannot speciate the gas sample, and therefore, there is still a need to have a GC installed in order to determine a total hydrocarbon content and/or a molecular weight of the flare vent gas.